How Much Concrete Do I Need?
Concrete quantity depends on the volume of your project — length × width × thickness for slabs, or the cylinder volume for post holes. Here is how to calculate it.
🧮Concrete Calculator — bags for slabs, post holes, and footings
Enter your exact dimensions for a personalized estimate →
The quick formula
Slab: Volume (cu ft) = Length × Width × (Thickness ÷ 12)
Post hole: Volume (cu ft) = π × (Radius ft)² × (Depth ÷ 12)
Bags = Volume (cu ft) × (1 + 0.10 waste) ÷ Bag yield, rounded up
Bag yield reference (Quikrete)
| Bag size | Yield per bag | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags |
| 50 lb | 0.375 cu ft | 72 bags |
| 60 lb | 0.45 cu ft | 60 bags |
| 80 lb | 0.60 cu ft | 45 bags |
Thickness guide
| Application | Thickness |
|---|---|
| Light-duty walkway | 3–4 inches |
| Patio / residential slab | 4 inches |
| Driveway | 5–6 inches |
| Structural footing | per engineer specs |
Bags for common slab sizes (80 lb bags, 4” thick, with 10% waste)
| Slab size | Cu ft | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 ft | 5.3 | 10 | 13 |
| 6 × 6 ft | 12.0 | 22 | 30 |
| 8 × 8 ft | 21.3 | 40 | 53 |
| 10 × 10 ft | 33.3 | 62 | 82 |
| 10 × 20 ft | 66.7 | 123 | 164 |
| 12 × 12 ft | 48.0 | 88 | 118 |
Post hole reference (per hole, 10% waste, 80 lb bags)
| Hole diameter | 24” deep | 36” deep | 48” deep |
|---|---|---|---|
| 6” | 1 bag | 2 bags | 2 bags |
| 8” | 2 bags | 3 bags | 4 bags |
| 10” | 3 bags | 4 bags | 6 bags |
| 12” | 4 bags | 6 bags | 8 bags |
Use the Concrete Calculator and choose “Post Holes” to enter your exact diameter, depth, and number of holes.
Frequently Asked Questions
How many 80 lb bags of concrete do I need for a 10×10 slab at 4 inches?
A 10×10 ft slab at 4 inches thick has a volume of 33.3 cubic feet. Adding 10% waste gives 36.7 cu ft. At 0.60 cu ft per 80 lb bag, you need 62 bags.
How many bags of concrete make a cubic yard?
80 lb bags: 45 bags. 60 lb bags: 60 bags. 50 lb bags: 72 bags. 40 lb bags: 90 bags. These are based on Quikrete manufacturer specifications.
How thick should a concrete slab be?
Standard residential slabs are 4 inches thick. Driveways and areas with vehicle traffic should be 5–6 inches. Light-duty walkways can be 3–4 inches. Structural footings vary by design.
How much concrete do I need for a fence post hole?
A 6-inch diameter hole that is 36 inches deep needs about 0.59 cubic feet of concrete, or 1 bag of 80 lb Quikrete per hole. Use the Concrete Calculator with the Post Hole option for your specific dimensions.
How do I measure for a concrete footing?
Measure the rectangular footprint (length × width) and depth of the footing. Multiply length × width × depth (all in feet) to get cubic feet, divide by 27 for cubic yards, then divide by bag yield for bag count.